ANKARA, March 6 (Reuters) - Turkey's constitutional court backed former army chief Ilker Basbug's bid for release from a life jail sentence on Thursday, increasing disarray in the trial of coup plots against Prime Minister Tayyip Erdogan who is now battling a new foe.

The ruling paves the way for his possible release by a lower court and could be a precedent for more than 200 other defendants jailed for their alleged roles in the 'Ergenekon' conspiracy against Erdogan's government.

Basbug has been held in Silivri prison near Istanbul for 26 months in connection with the 'Ergenekon' case, a trial which helped tame Turkey's once all-powerful military.

The five-year trial, which reached a verdict last August, was key to a decade-long battle between Erdogan's Islamist-rooted AK Party and a secularist establishment that had led modern Turkey from its foundation by Mustafa Kemal Ataturk.

Erdogan is now engaged in a power struggle with a former ally, U.S.-based Islamic cleric Fethullah Gulen, whom he accuses of using his influence in the judiciary and police to engineer a graft probe targeting the government. Gulen denies the charge.

The constitutional court said the failure of the lower court to publish its detailed verdict on the case and send it to the appeals court had violated a clause concerning personal freedom.

"It was decided ... to send to the (lower) court a request to do what is necessary in ruling on the applicant's release demand," said the ruling on the court website. It was not clear why the detailed verdict had still not been completed.

Parliamentary speaker Cemil Cicek, from the ruling AK Party, hailed the ruling as a triumph of judicial reforms which have been pushed through parliament in recent years.

"This means that subjects such as long detention periods, rights violations and the failure to try people fairly can now be resolved in Turkey," he told Reuters.

TAMING THE ARMY

Erdogan is widely believed to have relied heavily on Gulen's influence in breaking the power of the army, which carried out three coups in Turkey between 1960 and 1980 and forced an Islamist-led government from power in 1997.

Gulen's critics say his followers were instrumental in bringing to court the 'Ergenekon' case and another alleged conspiracy, dubbed 'Sledgehammer'.

In the 'Ergenekon' case, a network of secular nationalists was alleged to have planned killings and bombings to trigger a coup. Defendants included army officers, politicians, academics and journalists. They denied the charges.

Military commanders in the 'Sledgehammer' case were accused of plotting to bomb mosques and stoke conflict with Greece with the same goal. More than 300 officers were convicted in that case in September 2012 and the appeals court upheld most of the convictions.

Critics said the charges in both cases were trumped up and aimed at stifling opposition in Turkey. They accused Erdogan of trying to exert political influence over the courts.

Erdogan was initially a strong advocate of the cases, but in the last two years has become increasingly critical of state prosecutors, expressing disquiet at the length of time the defendants have been held in custody.

Celal Ulgen, a lawyer for many of the defendants, said the court ruling could lead to the release of the other prisoners, but did not expect a decision on Basbug's case on Thursday.

"It could set an example for hundreds of prisoners. Those tried under Ergenekon have a solid reason to apply for the same procedure. I am sure they will," he told Reuters.

Within weeks of the corruption investigation emerging, Erdogan mooted in January the retrial of those convicted of trying to overthrow him in an apparent bid to discredit those in the judiciary he saw as concocting the scandal.

His top adviser Yalcin Akdogan said the military had been the victim of a plot and the army subsequently filed a criminal complaint over the cases, arguing that evidence against serving and retired officers had been fabricated.

Last month, the government pushed through parliament a law completing the abolition of the special authority courts which tried the Ergenekon defendants and several hundred military officers in the separate Sledgehammer conspiracy.

President Abdullah Gul approved that law on Thursday. (Additional reporting by Ece Toksabay; Writing by Daren Butler; Editing by Gareth Jones)

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